Stock-based Compensation - Stock Option Activity (Details) - Service Based Stock Options - $ / shares	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Shares
Beginning balance (in shares)	6,182,873
Granted (in shares)	2,013,173	358,192
Exercised (in shares)	(93,587)
Expired (in shares)	0
Forfeited (in shares)	(119,089)
Ending balance (in shares)	7,983,370		6,182,873
Exercisable (in shares)	4,208,609
Vested and expected to vest (in shares)	7,983,370
Weighted average exercise price per share
Beginning balance (usd per share)	$ 10.60
Granted (usd per share)	7.73
Exercised (usd per share)	3.98
Expired (usd per share)	0.00
Forfeited (usd per share)	11.34
Ending balance (usd per share)	9.94		$ 10.60
Exercisable (usd per share)	9.07
Vested and expected to vest (usd per share)	$ 9.94
Weighted average remaining contractual life
Options outstanding, Weighted average remaining contractual life	6 years 10 months 20 days		6 years 8 months 1 day
Options exercisable, Weighted average remaining contractual life	5 years 5 days
Vested and expected to vest, Weighted average remaining contractual life	6 years 10 months 20 days

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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